A University of Texas at Dallas researcher has published a series of papers about her investigations into the nature of primary muscle tension dysphonia (pMTD), a voice disorder that has proved hard to quantify or link to a cause.
Dr. Adrianna Shembel, assistant professor of speech, language, and hearing in the School of Behavioral and Brain Sciences (BBS), has devised new metrics and methods to track laryngeal muscle movement patterns in people with the disorder, common among those in vocally demanding professions, such as education or performance.
Two articles published online June 13 in The Laryngoscope link the cause of pMTD to differences in how individuals with and without a diagnosis of pMTD perceive sensations in the vocal system during voice use; these may play a larger role in voice issues than increased muscle tension.
“This is an extremely understudied and underdiagnosed condition,” said Shembel, who has a joint appointment as an assistant professor of otolaryngology-head and neck surgery at UT Southwestern Medical Center. “In the U.S., an average of 40% of voice clinician caseloads consist of these functional voice disorders, where the larynx appears and moves normal.”
Muscle tension dysphonia has traditionally been attributed to an imbalance in the vocal muscle contractions and breathing patterns that create a person’s voice. Symptoms include pain or tiredness after sustained speech, or reduced voice quality.
“Patients describe severe issues with vocal strain, effort and fatigue. Many patients also have hoarse and rough vocal qualities and difficulties with vocal projection,” Shembel said. “It often occurs with a change in vocal demands: We see it a lot in professional voice users, like instructors, performers and worship leaders. They feel as though they can’t get through a full day at their job.”
Shembel’s research focuses on determining the cause of pMTD.
“We’re doing evaluations where everything seems to be functioning just fine,” Shembel said. “But they’re still really struggling. This can be a real burden on people’s livelihoods and quality of life.”
In one study, Shembel set out to develop physiologic metrics to quantify tension and strain in the muscles of the vocal folds and those that suspend and stabilize the larynx. Sixty-five individuals with vocally demanding occupations were recruited for the study. Half had been diagnosed with pMTD but had not begun voice therapy.
“We first tried shear wave elastography, an ultrasound method using small acoustic pulses sent to the extrinsic muscles that connect the jaw and neck to the larynx,” Shembel said. “The speed at which those pulses bounce back indicates the tissue’s stiffness, like a ball bouncing harder off a wall than a pillow.”
The researchers found no difference in stiffness between those diagnosed with pMTD and the control group of healthy occupational voice users, suggesting tension in those muscles was similar between groups with and without pMTD.
“We think patients with pMTD are perceiving more vocal effort, strain and fatigue because their sensory system is less precise, less stable.”
Dr. Adrianna Shembel, assistant professor of speech, language, and hearing in the School of Behavioral and Brain Sciences
In the second study involving 30 participants, half of whom had a pMTD diagnosis, 16 markers were placed on different anatomical landmarks on their chins and necks. Movements across these regions were tracked during four voice and speech tasks using two 3D cameras.
From this experiment, Shembel developed a method to measure the velocity of movement patterns as a physiologic proxy to study patterns in the underlying muscles that suspend and support the larynx. The findings showed few differences between the two cohorts, with motion capture footage indicating only increased variability in movement in the pMTD patients.
“All of the self-perceptual measures — vocal effort, discomfort, strain and fatigue — are significant, but the physiological metrics we have objectively measured were no different,” she said. “This led us to guess that this could be in part a sensory disorder — a change in how patients with pMTD perceive sensation.”
In an earlier study published online April 1 in the Journal of Voice on how 100 people with various voice disorders perceive their bodies, Shembel and her colleagues found that patients with pMTD who reported more vocal impact and fatigue also had less sensory perception awareness.
“We think patients with pMTD are perceiving more vocal effort, strain and fatigue because their sensory system is less precise, less stable,” she said.
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Shembel is now studying patients for sensory differences in and around the larynx — differences that could be either a vocal sensation or a more central sensitivity.
“People with muscle tension dysphonia have a higher prevalence of conditions like fibromyalgia, irritable bowel syndrome and other central sensitivity disorders,” she said. “It could be caused by chemoreceptor changes at the periphery, or it could be more of a central nervous system hypersensitivity.”
This fall, Shembel and members of her Integrative Laryngeal BioPhysiology Lab will present their findings at both the ASHA (American Speech-Language-Hearing Association) Convention in Boston and The Fall Voice Conference in Washington, D.C.
For now, standard treatment for pMTD focuses on voice therapy techniques that restore muscle balance by exercising the coordinated processes through which the body creates the voice.
“Methods such as resonant voice therapy change the biomechanics of how we produce sound, using more air flow and less muscle activation to do the work,” Shembel said. “That therapy may also fine-tune the somatosensory system. We can also do manual manipulation that could change the perception of sensation that people are experiencing.”
Shembel was corresponding author of all three studies. Other authors affiliated with UT Dallas included Robert Morrison BS’15, MS’18, PhD’21, Sarah McDowell MS’23 and Julianna Comstock Smeltzer MS’23; Dr. Xiaohu Guo, professor of computer science in the Erik Jonsson School of Engineering and Computer Science; computer science doctoral student Steven Hogue; and former assistant professor of mathematical sciences Dr. Sy Han Chiou.
UT Southwestern contributors to the work included Dr. Ted Mau, professor of otolaryngology-head and neck surgery; Dr. David Fetzer, assistant professor of radiology; and advanced practice sonographer Amber Patterson-Lachowicz.
Shembel’s work is supported by a grant (R21DC019207) from the National Institute on Deafness and Other Communication Disorders, a component of the National Institutes of Health, and the UT Dallas Seed Program for Interdisciplinary Research.